The present invention relates to processes for producing III-N bulk crystals from a gas or vapor phase, an well as to a process for producing free-standing III-N substrates which are obtained from the III-N bulk crystals. The term “III-N” denotes a III-N compound, where III denotes at least one element of group III of the periodic table of elements, selected from aluminum, gallium and indium (in the following sometimes abbreviated by (Al, Ga, In)), and N denotes nitrogen. The III-N bulk crystal and the free-standing III-N substrate comprise the III-N compound as the main material, and they are respectively preferably composed essentially of or completely of the III-N compound, optionally together with impurities. The invention further relates to III-N bulk crystals and freestanding III-N substrates as such, which are advantageously obtainable by such processes. The free-standing III-N substrates are particularly suitable as substrates for the manufacture of optoelectronic and electronic apparatuses.
In industrial applications, components or devices for (Al, Ga, In) N-based light-emitting diodes or laser diodes have conventionally been grown on or above foreign substrates, such as Al2O3 (sapphire) or SiC. The drawbacks relating to crystal quality and consequently lifetime and efficiency of the component or the device, which result from the use of foreign substrates, can be mitigated through growth on III-N substrates, such as e.g. (Al, Ga) N substrates. Until now, however, such substrates have not been available in sufficient quality. This is largely caused by difficulties in conventional bulk growth technologies due to the extremely high steady state vapor pressure of nitrogen over III-N compounds at typical growth temperatures. The growth of bulk material under high pressure was described by Porowski (MRS Internet J. Nitride Semiconduct. Res. 4S1, 1999, G1.3). This process provides qualitatively valuable GaN bulk material, but has the disadvantage that, up to now, only small GaN substrates with a surface area of maximally 100 mm2 can be produced thereby. Furthermore, the production process requires a long production time in comparison with other processes and is technologically laborious and cost-intensive due to the extremely high growth pressures.
A further method consists in the growth of III-N materials on a foreign substrate from the gas phase or vapor phase, followed by separation from the foreign substrate. For the production of thick free-standing layers of III-N, such as GaN, it is known e.g. from M. Kelly et al. (Jpn. J. Appl. Phys. Vol. 38, 1999, pp. L217-L219), “Large Free-Standing GaN Substrates by Hydride Vapor Phase Epitaxy and Laser Induced Lift-Off”, to separate a thick GaN layer, which had been previously grown on a substrate made of sapphire (Al2O3) by means of hydride vapor phase epitaxy (HVPE), from the sapphire substrate. For this purpose, the document describes irradiating the GaN-deposited sapphire substrate with a laser, with the result that the GaN layer is locally thermally decomposed at the interface with the sapphire substrate, and thereby lifting off from the sapphire substrate. Alternative separation methods include wet chemical etching (for example of GaAs; see K. Motoki et al. Jap. J. Appl. Phys. Vol. 40, 2001, pp. L140-L143), dry chemical etching (for example of SiC; Yu. Melnik et al., Mat. Res. Soc. Symp. Proc. Vol. 482, 1998, pp. 269-274) or mechanical lapping (for example of sapphire; see H.-M. Kim et al., Mat. Res. Soc. Symp. Proc. Vol. 639, 2001, pp. G6.51.1-G6.51.6) of the substrate. Drawbacks of the aforementioned methods lie on the one hand in the relatively high costs due to the complex technologies for substrate separation, and on the other hand in the fundamental difficulty to produce III-N materials having a homogenously low defect density.
The growth of thick III-N bulk crystals (boules) on a III-N substrate by vapor phase epitaxy and the subsequent separation of the bulk crystals to obtain single III-N substrates by a sawing process offer an alternative to the aforementioned processes. Such a process was described by Vaudo et al. (U.S. Pat. No. 6,596,079). HVPE was chosen as the preferred growth method; as preferred boule crystal lengths, values of >1 mm, 4 mm or 10 mm are indicated. Vaudo et al, further describe inter alia how III-N substrates are obtained from the bulk crystal by wire sawing or further treatment steps, such as chemical/mechanical polishing, reactive ion etching or photo-electrochemical etching. Furthermore, III-N bulk crystals and substrates produced by the aforementioned technique are also mentioned in an international patent application of Vaudo et al. (WO 01/68955 A1).
Melnik et al. describe a process for the growth of GaN- (U.S. Pat. No. 6,616,757) or AlGaN-bulk crystals (US 2005 0212001 A1) having crystal lengths greater than 1 cm. The process basically consists of the steps: growth or a single crystalline (Al)GaN layer on a substrate, removing the substrate and washing the (Al)GaN bulk crystal on the single crystal (Al)GaN layer. As a preferred method a HVPE process with a specific reactor configuration is mentioned. Further, Melnik et al. describe in a US application (US 2005-0164044 A1) and in U.S. Pat. No. 6,936,357 GaN- and AlGaN-bulk crystals having various characteristics, such as dimensions, dislocation densities or full widths at half maximum of X-ray rocking curves.